Seeing an enemy TAG across the table can be a scary sight. When you finally destroy the Nomads Iguana TAG, your work is not done, however. It ejects its pilot, which is a scary Heavy Infantry model in its own right!
The tiny scrapes and scratches are achieved by making very fine lines using the highlight colors of whatever area is getting scratched up. So, on these metal bits, I am using VMC 155 Light Grey and pure white to make diagonal and criss-crossing scratch marks.
It requires a very good brush that comes to an excellent point (I recommend any brush made of Kolinsky Sable, such as the Windsor & Newton Series 7 Rounds). It also requires the paint to be thin enough to flow easily from brush to model, but not so thin that it is transparent or runs. Also, very light pressure is used - the harder that you press with the brush, the wider the line you make gets. The faster you paint the line, the straighter it will be.